Hydrogen-producing fuel processing systems include a plurality of system components, and each may be operated in a distinct chemical and/or physical environment. This may include differences in the temperature of the components, differences in the pressure of the gas streams contained within the components, and/or differences in the chemical composition of the materials contained therein, the streams that are supplied thereto, and/or the streams that are removed from the components. The chemical and/or physical environment of the components also may vary as a function of the operational state or status of the fuel cell system.
When operated continuously and/or under steady-state conditions, the chemical and/or physical environment within the various components of the hydrogen-producing fuel processing system may tend to be relatively constant. However, when operated intermittently, such as when the hydrogen-producing fuel processing system forms a portion of a fuel cell system that may be utilized as an auxiliary or backup power system, the transitions between the various operational states and/or the associated changes in the environment of the system components may lead to a decrease in system performance. This decrease in performance may become more significant as the length of the period of inactivity increases.
As an illustrative, non-exclusive example, hydrogen-producing fuel processing systems for use in fuel cell applications, as well as other applications that require a high-purity hydrogen gas stream, may include a separation assembly designed to separate the mixed gas (i.e., reformate) stream produced by a hydrogen generation assembly into a product hydrogen stream and a byproduct stream. The product hydrogen stream includes hydrogen gas at a required purity for use in the fuel cell stack, and the byproduct stream includes other gasses and/or contaminants that were present in the reformate stream. The separation assembly may utilize a variety of technologies, with hydrogen-selective, or hydrogen-permeable, membranes being an example of a useful technology for purifying hydrogen gas.
Hydrogen-selective membranes, such as which may be formed from palladium or a palladium alloy, are permeable to hydrogen gas but not to other gasses. Such hydrogen-selective membranes may be effective at removing impurities, such as carbon monoxide and other gasses, from the reformate stream during operation of the hydrogen-producing system. However, these membranes may not be as effective when the hydrogen-producing system is only operated intermittently to produce hydrogen gas, especially if the hydrogen-selective membrane is maintained at elevated temperatures and allowed to come into contact with oxides and/or contaminants during the periods of inactivity. In particular, intermittently used hydrogen-selective membranes may exhibit reduced permeability for hydrogen gas, resulting in low flow rates of the product hydrogen stream and a corresponding increase in the flow rate of the byproduct stream in the corresponding hydrogen-producing system. Thus, there exists a need for systems and methods for maintaining the hydrogen permeability of hydrogen-selective membranes during periods of inactivity in which the membranes are maintained at conditions where oxidation of the membranes may occur and/or in which the membranes may be exposed to contaminants.